As a way to combat global warming, introduction of new energy such as solar photovoltaic power generation and wind power generation has been promoted in recent years throughout the world. Since outputs of these power generations are affected by the weather, it is predicted that introduction on a large scale will cause problems with operation of power systems such as difficulty in maintaining frequencies and voltages. As a way to solve such problems, installation of large-capacity storage batteries for smoothing output variations, storing surplus power, and load leveling is expected.
A redox flow battery is one of large-capacity storage batteries. A redox flow battery is charged and discharged by supply of a positive electrode electrolyte and a negative electrode electrolyte to a battery element having a membrane interposed between a positive electrode and a negative electrode. An aqueous solution containing a metal ion having a valence which changes by oxidation-reduction is typically used as the electrolytes. Typical redox flow batteries include an iron-chromium-based redox flow battery containing an iron ion for a positive electrode and a chromium ion for a negative electrode, and an all-vanadium redox flow battery containing a vanadium ion for both the positive and negative electrodes (for example, Japanese Patent Laying-Open No. 2001-043884 (Patent Document 1) and Japanese Patent Laying-Open No. 2006-147374 (Patent Document 2)).